;; This module implements lazy SXPath evaluation over lazy SXML documents
;
; This software is in Public Domain.
; IT IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND.
;
; Please send bug reports and comments to:
;   lizorkin@hotbox.ru    Dmitry Lizorkin
;
; In a lazy SXML document, each node may be a promise. If forced, the promise
; results into an SXML node or a nodeset. For a nodeset, its members are SXML
; nodes and promises in turn.
; With every promise forced, a lazy SXML document must conform to SXML
; Specification. In particular, an attribute node must occur before any child
; nodes, attribute value must be atomic, etc.
;
; SXPath evaluation is lazy in that it results to a nodeset whose last member
; may be a promise. Such a nodeset with a promise as its last member denotes
; the first portion of the result. If the promise is forced, it is evaluated
; into another nodeset, which corresponds to the next portion of the result.
; SXPath evaluator returns the result in portions when some branch in the
; document is to be forced in order to obtain the next part of the result.
; However, a portion that is not the last one, must contain at least one result
; node. To fulfill this requirement, branches of the document may be forced
; until at least a result node for a portion is obtained.

; Implement 'or' as a function, so that we could 'apply' it
(define (lazy:or . args)
  (if (null? args) #f (or (car args) (apply lazy:or (cdr args)))))

;=========================================================================
; Misc helpers for working with a lazy nodeset

; Escaping the ## for some Scheme implementations
(cond-expand
 (gambit
  ; The following macro constructs Gambit-specific ids on the fly
  ; Borrowed from "http.scm"
  (define-macro (_gid id)
    (string->symbol (string-append "##" (symbol->string id))))
  )
 (chicken
  ; The following macro encapsulates the function ##sys#structure?
  ; Thanks to Thomas Chust and Felix Winkelmann for the explanation of
  ; qualified symbols in Chicken
  (define-macro (chk:sys-structure?)
    (string->symbol
     (string-append (string (integer->char 3)) "sys" "structure?")))
  )
 (else
  #t))

; Predicate for detecting a promise
; There is no such a predicate in R5RS, so different Scheme implementations
; use different names for this functionality
(define lazy:promise?
  (cond-expand
   (plt promise?)
   (bigloo
    procedure?   ; ATTENTION: returns #t in more general situations
    )
   (chicken
    ; Thanks to Zbigniew Szadkowski <zbigniewsz@gmail.com>
    ; for the insight of this function
    (lambda (p) ((chk:sys-structure?) p 'promise))
    )
   (gambit
    (_gid promise?)
    )
   (else
    (lambda (obj) #f)   ; ATTENTION: just makes the approach applicable for
                        ; conventional SXML documents
   )))

;-------------------------------------------------
; Lazy analogues for common list operations

; Checks whether the nodeset is empty
; Note that a promise can evaluate to an empty list, and thus a nodeset
; consisting of promises only may potentially be empty
(define (lazy:null? nodeset)
  (cond
    ((null? nodeset) #t)
    ((not (null? (filter   ; contains at least one non-promise
                  (lambda (node) (not (lazy:promise? node)))
                  nodeset)))
     #f)
    (else  ; all nodeset members are promises
     (let iter-promises ((nset nodeset))
       (cond
         ((null? nset) #t)
         ((lazy:null? (as-nodeset (force (car nset))))
          (iter-promises (cdr nset)))
         (else #f))))))

; Like conventional map, but applicable to a lazy nodeset
(define (lazy:map func nodeset)
  (cond
    ((null? nodeset)  ; iteration is over
     nodeset)
    ((lazy:promise? (car nodeset))
     (list
      (delay
        (lazy:map func
                  (append (as-nodeset (force (car nodeset)))
                          (cdr nodeset))))))
    (else  ; the first member is a node
     (cons (func (car nodeset))
           (lazy:map func (cdr nodeset))))))

; Lazy analogue for filter
(define (lazy:filter func nodeset)
  (cond
    ((null? nodeset)  ; iteration is over
     nodeset)
    ((lazy:promise? (car nodeset))
     (list
      (delay
        (lazy:filter func
                  (append (as-nodeset (force (car nodeset)))
                          (cdr nodeset))))))
    ; the first member is a node
    ((func (car nodeset))
     (cons (car nodeset)
           (lazy:filter func (cdr nodeset))))
    (else  ; the first member doesn't satisfy the predicate
     (lazy:filter func (cdr nodeset)))))

; Like conventional car, but for a lazy nodeset
(define (lazy:car nodeset)
  (cond
    ; Checking for a safe variant
    ;((null? nodeset)  ; failed
    ; #f)
    ((lazy:promise? (car nodeset))
     (let ((nset-car (force (car nodeset))))
       (lazy:car
        ((if (nodeset? nset-car) append cons)
         nset-car (cdr nodeset)))))
    (else
     (car nodeset))))

; Like conventional cdr
(define (lazy:cdr nodeset)
  (if
   (lazy:promise? (car nodeset))
   (let ((nset-car (force (car nodeset))))
     (lazy:cdr
      ((if (nodeset? nset-car) append cons)
       nset-car (cdr nodeset)))))
  (cdr nodeset))

; Like conventional length, but for a lazy nodeset
; ATTENTION: it has to force all the nodeset members in order to determine
; the length properly
(define (lazy:length nodeset)
  (cond
    ((null? nodeset) 0)
    ((lazy:promise? (car nodeset))
     (let ((nset-car (force (car nodeset))))
       (lazy:length
        ((if (nodeset? nset-car) append cons)
         nset-car (cdr nodeset)))))
    (else
     (+ 1 (lazy:length (cdr nodeset))))))

;-------------------------------------------------

; Converts the lazy result into a list, by forcing all the promises one by one
(define (lazy:result->list nodeset)
  (let iter-nset ((nset nodeset)
                  (res '()))
    (cond
      ((null? nset)  ; finished scanning
       (reverse res))
      ((lazy:promise? (car nset))
       (iter-nset (append (as-nodeset (force (car nset))) (cdr nset))
                  res))
      (else  ; the first member is a node
       (iter-nset (cdr nset)
                  (cons (car nset) res))))))

; Converts the lazy node to SXML, by forcing all of its descendants
; The node itself is not a promise
(define (lazy:node->sxml node)
  (letrec
      ((force-nodeset
        (lambda (nodeset)
          (cond
            ((null? nodeset) nodeset)
            ((lazy:promise? (car nodeset))
             (let ((nset-car (force (car nodeset))))               
               (force-nodeset
                ((if (nodeset? nset-car) append cons)
                 nset-car (cdr nodeset)))))
            (else
             (cons (lazy:node->sxml (car nodeset))
                   (force-nodeset (cdr nodeset))))))))
    (if
     (or (not (pair? node))
         (null? ((sxml:descendant lazy:promise?) node)))
     node     ; will not make a copy of the node
     (cons (car node) (force-nodeset (cdr node))))))

; Reaches the root of the root of the contextset
; Result: singleton nodeset
(define (lazy:reach-root contextset)
  (letrec
      ((find-root
        (lambda (src prev-result)
          (let loop ((src src)
                     (res '())
                     (prev-result prev-result))
            (cond
              ((null? src)  ; nothing more to do
               (reverse res))
              ((lazy:promise? (car src))
               (if
                (null? res)  ; need to force this
                (loop (append (as-nodeset (force (car src)))
                              (cdr src))
                      res prev-result)
                (reverse
                 (cons (delay (find-root src prev-result))
                       res))))
              (else  ; (car src) is the ordinary node
               (let ((rt (if (sxml:context? (car src))
                             (draft:list-last
                              (sxml:context->ancestors-u (car src)))
                             (car src))))
                 (loop (cdr src)
                       (if
                        (memq rt prev-result)  ; already returned
                        res (cons rt res))
                       (cons rt prev-result)))))))))
    (find-root contextset '())))

; Analogue for draft:contextset->nodeset for the lazy case
(define (lazy:contextset->nodeset obj)
  (letrec
      ((iter-nset
        (lambda (nset)
          (cond
            ((null? nset) nset)
            ((lazy:promise? (car nset))
             (list
              (delay (iter-nset (append (as-nodeset (force (car nset)))
                                        (cdr nset))))))
            (else  ; (car nset) is a node
             (cons
              (sxml:context->node (car nset))
              (iter-nset (cdr nset))))))))
  (if
   (nodeset? obj)
   (iter-nset obj)
   obj)))

; Lazy analogue for draft:recover-contextset
(define (lazy:recover-contextset nodeset root-node num-anc)
  (cond
    ((null? nodeset)  ; nothing more to do
     '())
    ((lazy:promise? (car nodeset))
     (delay (lazy:recover-contextset
             (append (as-nodeset (force (car nodeset)))
                     (cdr nodeset))
             root-node num-anc)))
    (else  ; (car nodeset) is a common node
     (cons
      (draft:smart-make-context
       (car nodeset)
       (((sxml:ancestor (lambda (x) #t)) root-node) (car nodeset))
       num-anc)
      (lazy:recover-contextset (cdr nodeset) root-node num-anc)))))

; Makes a context-set from a nodeset supplied, with the num-anc required
; Members of the nodeset are known to be descendants-or-selves of
; (map sxml:context->node context-set)
(define (lazy:find-proper-context nodeset context-set num-anc)
  (let* ((descend (lazy:descendant-or-self sxml:node? num-anc))
         (possible-ancestors
          (map
           cdr  ; ignore starting '*CONTEXT* for a faster search
           (map-union
            (lambda (node)
              ; Has to be evaluated in the active manner, since all of the
              ; candidates generally have to be scanned
              (lazy:result->list (descend node)))
            ;(lazy:result->list ancestors-set)
            (map-union
             sxml:context->ancestors
             (lazy:result->list context-set))))))
    (let iter-nset ((nodeset nodeset)
                    (res '()))   ; DL: was: res
      (cond
        ((null? nodeset)  ; scanning is over
         (reverse res))
        ((lazy:promise? (car nodeset))
         (if (null? res)  ; result is still null => have to force
             (iter-nset (append (as-nodeset (force (car nodeset)))
                                (cdr nodeset))
                        res)
             (reverse
              (cons
               (delay (iter-nset (append (as-nodeset (force (car nodeset)))
                                         (cdr nodeset))
                                 '()))
               res))))
        ((sxml:context? (car nodeset))  ; already a context
         (iter-nset (cdr nodeset)
                    (cons (car nodeset) res)))
        ((assq (car nodeset) possible-ancestors)
         => (lambda (ancestors)
              (iter-nset (cdr nodeset)
                         (cons
                          (draft:make-context
                           (car ancestors)  ; = (car nodeset)
                           (cdr ancestors))
                          res))))
        (else  ; this is a newly constructed node
         ; Keep it as is
         (iter-nset (cdr nodeset)
                    (cons (car nodeset) res)))))))


;=========================================================================
; Axes

; A helper that tests sibling nodes with respect to the test-pred? and returns
; them in the lazy manner. Each of the siblings may be a promise.
; This function is applied by axes: child, attribute, namespace,
; following-sibling, preceding-sibling
(define (lazy:output-siblings test-pred? siblings ancestors)
  (letrec
      ((iterate-siblings
        (lambda (src)
          (let loop ((src src) (res '()))
            (cond
              ((null? src)  ; iteration is over
               (reverse res))
              ((lazy:promise? (car src))               
                (reverse  ; otherwise - return the result with a promise
                 (cons
                  (delay
                    (iterate-siblings
                     (append (as-nodeset (force (car src))) (cdr src))))
                  res)))
              (else  ; the first src is a node
               (loop (cdr src)
                     (if (test-pred? (car src))
                         (cons
                          (if (null? ancestors)  ; don't construct context
                              (car src)
                              (draft:make-context (car src) ancestors))
                          res)
                         res))))))))
    (iterate-siblings siblings)))

; Returns nodeset membert that are following siblings of the given node
; Nodeset members may be promises
(define (lazy:find-foll-siblings node nodeset)
  (cond
    ((null? nodeset)  ; not found
     '())
    ((lazy:promise? (car nodeset))
     (lazy:find-foll-siblings
      node
      (append (as-nodeset (force (car nodeset)))
              (cdr nodeset))))
    ; (car nodeset) an ordinary node
    ((eq? node (car nodeset))
     (cdr nodeset))
    (else
     (lazy:find-foll-siblings node (cdr nodeset)))))

; Returns nodeset members that are preceding siblings of the given node
; Nodeset members may be promises
(define (lazy:find-prec-siblings node nodeset)
  (let loop ((nodeset nodeset)
             (res '()))
    (cond
      ((null? nodeset)  ; not found
       '())
      ((lazy:promise? (car nodeset))
       (loop
        (append (as-nodeset (force (car nodeset)))
                (cdr nodeset))
        res))
      ; the first member in a nodeset an ordinary node
      ((eq? node (car nodeset))
       res)
      (else
       (loop (cdr nodeset)
             (cons (car nodeset) res))))))

;-------------------------------------------------
; Axis functions
; They have the signature of:
;  test-pred? [num-ancestors] -> Node -> Nodeset
; Note that each axis function produces the function to be applied to a single
; _node_, not to a nodeset

; Ancestor axis
; It should be noted that ancestors of the context node are already forced
(define (lazy:ancestor test-pred? . num-ancestors)
  (let* ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (if
       (sxml:context? node)
       (let loop ((ancs-to-view (sxml:context->ancestors-u node))
                  (res '()))
         (cond
           ((null? ancs-to-view)  ; processed everyone
            (reverse res)  ; reverse document order required
            )
           ((test-pred? (car ancs-to-view))  ; can add it to result
            (loop
             (cdr ancs-to-view)
             (cons
              (draft:smart-make-context
               (car ancs-to-view) (cdr ancs-to-view) num-anc)
              res)))
           (else  ; current node doesn't satisfy the predicate
            (loop (cdr ancs-to-view) res))))
       '()  ; no ancestors
       ))))

; Ancestor-or-self axis
; It should be noted that ancestors of the context node are already forced
(define (lazy:ancestor-or-self test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (cond
        ((sxml:context? node)
         (let loop ((ancs-to-view (sxml:context->content-u node))
                    (res '()))
           (cond
             ((null? ancs-to-view)  ; processed everyone
              (reverse res)  ; reverse document order required
              )
             ((test-pred? (car ancs-to-view))  ; can add it to result
              (loop
               (cdr ancs-to-view)
               (cons
                (draft:smart-make-context
                 (car ancs-to-view) (cdr ancs-to-view) num-anc)
                res)))
             (else  ; current node doesn't satisfy the predicate
              (loop (cdr ancs-to-view) res)))))
        ; ordinary SXML node
        ((test-pred? node)  ; satisfies the predicate
         (list node))
        (else
         '())))))

; Attribute axis
(define (lazy:attribute test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (letrec
        ((find-attr-node
          ; Either returns an attribute node, or #f
          ; Nodeset members may be promises
          (lambda (nodeset)
            (cond
              ((null? nodeset)  ; failed
               #f)
              ((lazy:promise? (car nodeset))
               (find-attr-node
                (append (as-nodeset (force (car nodeset)))
                        (cdr nodeset))))
              ; (car nodeset) an ordinary node
              (((ntype?? '@) (car nodeset))
               (car nodeset))
              (else #f)))))
      (lambda (node)  ; not a nodeset
        (cond
          ((not (pair? node)) '())   ; no attributes
          ; (car node) is always a symbol
          ((sxml:context-u? node)  ; a context node
           (let ((attr-node (find-attr-node (sxml:context->node-u node))))
             (if (not attr-node)  ; not found
                 '()
                 ((lazy:child test-pred? num-anc)
                  (if (and num-anc (zero? num-anc))
                      attr-node
                      (draft:make-context
                       attr-node (sxml:context->content-u node)))))))
          (else  ; an ordinary node, and is a pair
           (let ((attr-node (find-attr-node node)))
             (if (not attr-node)  ; not found
                 '()
                 ((lazy:child test-pred? num-anc)
                  (if (and num-anc (zero? num-anc))
                      attr-node                      
                      (draft:make-context attr-node (list node))))))))))))           

; Child axis
(define (lazy:child test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))    
    (lambda (node)  ; not a nodeset
      (cond
        ((not (pair? node))  ; no children
         '())
        ; (car node) is always a symbol
        ((sxml:context-u? node)  ; a context node
         (let ((this (sxml:context->node-u node)))
           (if
            (or (not (pair? this))
                (memq (car this) '(*PI* *COMMENT* *ENTITY*)))
            '()  ; no children
            (lazy:output-siblings
             test-pred?
             (cdr this)  ; gives its children
             (draft:list-head (sxml:context->content-u node) num-anc)))))
        ; an ordinary node, and is a pair
        ((memq (car node) '(*PI* *COMMENT* *ENTITY*))
         '())
        (else
         (lazy:output-siblings
          test-pred?
          (cdr node)  ; gives its children
          (if (and num-anc (zero? num-anc))
              '() (list node))))))))

; Descendant axis
(define (lazy:descendant test-pred? . num-ancestors)
  (let* ((num-anc (if (null? num-ancestors) 0 (car num-ancestors)))
         (child (lazy:child sxml:node? num-anc)))
    (lambda (node)  ; not a nodeset
      (let rpt ((res '())
                (more (child node)))        
        (cond
          ((null? more)  ; no more candidates         
           (reverse res))
          ((lazy:promise? (car more))  ; need to force it
           (reverse
            (cons
             (delay (rpt '()
                         (append (as-nodeset (force (car more)))
                                 (cdr more))))
             res)))
          (else  ; first in more is a node
           (rpt (if (test-pred? (sxml:context->node (car more)))
                    (cons (car more) res)
                    res)
                (append (child (car more)) (cdr more)))))))))

; Descendant-or-self axis
(define (lazy:descendant-or-self test-pred? . num-ancestors)
  (let* ((num-anc (if (null? num-ancestors) 0 (car num-ancestors)))
         (child (lazy:child sxml:node? num-anc)))
    (lambda (node)  ; not a nodeset
      (let rpt ((res '())
                (more (list node)))
        (cond
          ((null? more)  ; no more candidates         
           (reverse res))
          ((lazy:promise? (car more))  ; need to force it
           (reverse
            (cons
             (delay (rpt '()
                         (append (as-nodeset (force (car more)))
                                 (cdr more))))
             res)))
          (else  ; first in more is a node
           (rpt (if (test-pred? (sxml:context->node (car more)))
                    (cons (car more) res)
                    res)
                (append (child (car more)) (cdr more)))))))))

; Following axis
(define (lazy:following test-pred? . num-ancestors)
  (let* ((num-anc (if (null? num-ancestors) 0 (car num-ancestors)))
         (descend (lazy:descendant-or-self test-pred? num-anc)))
    (lambda (node)  ; not a nodeset
      (if
       (sxml:context? node)
       (let loop ((curr-node (sxml:context->node-u node))
                  (ancs-to-view (sxml:context->ancestors-u node))
                  (foll-siblings '())
                  (descendants '())
                  (res '()))
         (cond
           ((null? descendants)  ; candidates for result
            (cond
              ((null? foll-siblings)  ; no more siblings of the curr-node
               (if
                (null? ancs-to-view)  ; processed everyone                 
                (reverse res)
                (loop (car ancs-to-view)
                      (cdr ancs-to-view)
                      (lazy:find-foll-siblings
                       curr-node
                       (cdr  ; parent is an element => cdr gives its children
                        (car ancs-to-view)))
                      '()
                      res)))
              ((lazy:promise? (car foll-siblings))
               (reverse
                (cons
                 (delay
                   (loop curr-node ancs-to-view
                         (append (as-nodeset (force (car foll-siblings)))
                                 (cdr foll-siblings))
                         '() '()))
                 res)))
              (else  ; (car foll-siblings) is a node
               (loop curr-node ancs-to-view
                     (cdr foll-siblings)
                     (descend  ; descendants are currently null
                      (draft:smart-make-context
                       (car foll-siblings)
                       ancs-to-view num-anc))
                     res))))
           ((lazy:promise? (car descendants))  ; need to force descendant axis
            (reverse
             (cons
              (delay
                (loop curr-node ancs-to-view foll-siblings
                      (append (as-nodeset (force (car descendants)))
                              (cdr descendants))
                      '()))
              res)))
           (else  ; the first in descendants is a node
            (loop curr-node ancs-to-view foll-siblings
                  (cdr descendants) (cons (car descendants) res)))))
       '()  ; no following members       
       ))))

; Following-sibling axis
(define (lazy:following-sibling test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (if
       (and (sxml:context? node)
            (not (null? (sxml:context->ancestors-u node))))
       (lazy:output-siblings
        test-pred?
        (lazy:find-foll-siblings
         (sxml:context->node-u node)
         (cdr  ; parent is an element => cdr gives its children
          (car (sxml:context->ancestors-u node))))
        (draft:list-head
         (sxml:context->ancestors-u node) num-anc))
       '()  ; no parent => no siblings
       ))))

; Namespace axis
; Since a namespace axis somewhat redundant for SXML, we'll provide a
; not-very-effective implementation for it
(define (lazy:namespace test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))    
    (lambda (node)  ; not a nodeset
      (lazy:filter
       (lambda (context)
         (test-pred? (sxml:context->node context)))
       ((lazy:sxpath '(@@ *NAMESPACES* *) num-anc) node)))))

; Parent axis
; It should be noted that the parent of the context node is already forced
(define (lazy:parent test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (if
       (and (sxml:context? node)
            (not (null? (sxml:context->ancestors-u node)))
            (test-pred? (car (sxml:context->ancestors-u node))))
       (draft:smart-make-context
        (car (sxml:context->ancestors-u node))
        (cdr (sxml:context->ancestors-u node))
        num-anc)
       '()  ; no parent
       ))))

; Preceding axis
(define (lazy:preceding test-pred? . num-ancestors)
  (let* ((num-anc (if (null? num-ancestors) 0 (car num-ancestors)))
         (descend (lazy:descendant-or-self test-pred? num-anc)))
    (lambda (node)  ; not a nodeset
      (if
       (sxml:context? node)
       (let loop ((curr-node (sxml:context->node-u node))
                  (ancs-to-view (sxml:context->ancestors-u node))
                  (prec-siblings '())
                  (descendants '())
                  (res '()))
         (cond
           ((null? descendants)  ; candidates for result
            (cond
              ((null? prec-siblings)  ; no more siblings of the curr-node
               (if
                (null? ancs-to-view)  ; processed everyone                 
                (reverse res)
                (loop (car ancs-to-view)
                      (cdr ancs-to-view)
                      (lazy:find-prec-siblings
                       curr-node
                       (cdr  ; parent is an element => cdr gives its children
                        (car ancs-to-view)))
                      descendants  ; is null
                      res)))
              ((lazy:promise? (car prec-siblings))
               (reverse
                (cons
                 (delay
                   (loop curr-node ancs-to-view
                         (append (as-nodeset (force (car prec-siblings)))
                                 (cdr prec-siblings))
                         descendants  ; is null
                         '()))
                 res)))
              (else  ; (car prec-siblings) is a node
               (loop curr-node ancs-to-view
                     (cdr prec-siblings)
                     (reverse
                      (descend  ; descendants are currently null
                       (draft:smart-make-context
                        (car prec-siblings)
                        ancs-to-view num-anc)))
                     res))))
           ((lazy:promise? (car descendants))  ; need to force descendant axis
            (reverse
             (cons
              (delay
                (loop curr-node ancs-to-view prec-siblings
                      (append (reverse (as-nodeset (force (car descendants))))
                              (cdr descendants))
                      '()))
              res)))
           (else  ; the first in descendants is a node
            (loop curr-node ancs-to-view prec-siblings
                  (cdr descendants) (cons (car descendants) res)))))
       '()  ; no preceding members       
       ))))    

; Preceding-sibling axis
(define (lazy:preceding-sibling test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (if
       (and (sxml:context? node)                  
            (not (null? (sxml:context->ancestors-u node))))
       (draft:siblings->context-set
        ((sxml:filter test-pred?)
         (lazy:find-prec-siblings
          (sxml:context->node-u node)
          (cdr  ; parent is an element => cdr gives its children
           (car (sxml:context->ancestors-u node)))))
        (draft:list-head
         (sxml:context->ancestors-u node) num-anc))
       '()  ; no parent => no siblings
       ))))

; Self axis
; Shortens the context if it contains more nodes than specified by num-ancestor
; In most cases, this work can be considered redundant; however, it eliminates
; some classes of error that are hard to detect
(define (lazy:self test-pred? . num-ancestors)
  (let ((num-anc (if (null? num-ancestors) 0 (car num-ancestors))))
    (lambda (node)  ; not a nodeset
      (if (sxml:context? node)
          (if (test-pred? (sxml:context->node-u node))
              (list (draft:smart-make-context
                     (sxml:context->node-u node)
                     (sxml:context->ancestors-u node)
                     num-anc))
              '())
          (if (test-pred? node) (list node) '())))))

;-------------------------------------------------
; Making every axis consume a nodeset

; Given the axis of the form
;  Node -> Nodeset
; produces the axis of the form
;  Node|Nodeset -> Nodeset
(define (lazy:axis-consume-nodeset axis)
  (letrec
      ((iterate-nodeset
        ; candidates - candidate nodes for a result
        (lambda (src candidates)
          (let loop ((src src)
                     (candidates candidates)
                     (res '()))
            (cond
              ((null? candidates)  ; consume the following node from src
               (cond
                 ((null? src)  ; iteration is over
                  (reverse res))
                 ((lazy:promise? (car src))
                  (if
                   (null? res)  ; result is still empty, need to force src
                   (let ((src-car (as-nodeset (force (car src)))))
                     (cond
                       ((null? src-car)  ; a rare practical situation
                        (loop (cdr src) candidates res))
                       ((lazy:promise? (car src-car))  ; this shouldn't happen
                        (loop (append src-car (cdr src))
                              candidates
                              res))
                       (else  ; we can finally apply the axis
                        (loop (append (cdr src-car) (cdr src))
                              (axis (car src-car))  ; candidates are null
                              res))))
                   (reverse  ; otherwise - return the result with a promise
                    (cons
                     (delay (iterate-nodeset src candidates))
                     res))))
                 (else  ; (car src) is a node
                  (loop (cdr src)
                        (axis (car src))  ; candidates are null
                        res))))
              ((lazy:promise? (car candidates))               
               ; First candidate is a promise
               (if
                (null? res)  ; result is still empty, need to force candidate
                (let ((cand-car (as-nodeset (force (car candidates)))))
                  (cond
                    ((null? cand-car)  ; generally, (cdr candidates)=null
                     (loop src (cdr candidates) res))
                    ((lazy:promise? (car cand-car))  ; this shouldn't happen
                     (loop src
                           (append cand-car (cdr candidates))
                           res))
                    (else  ; add candidate to result
                     (loop src
                           (append (cdr cand-car) (cdr candidates))
                           (list (car cand-car))   ; res is null
                           ))))
                (reverse  ; otherwise - return the result with a promise
                 (cons
                  (delay (iterate-nodeset src candidates))
                  res))))
              (else   ; the first candidate is a node
               (loop src (cdr candidates)
                     (cons (car candidates) res))))))))
    (lambda (nodeset)  ; node or nodeset
      (cond
        ((null? nodeset)  ; nothing to do
         '())
        ((and (pair? nodeset) (symbol? (car nodeset)))  ; node
         (axis nodeset))
        (else  ; the general case
         (iterate-nodeset nodeset '()))))))


;=========================================================================
; Lazy analogues for "sxpath-ext.scm"

;-------------------------------------------------
; SXML counterparts to W3C XPath Core Functions Library

; The counterpart to XPath 'string' function (section 4.2 XPath Rec.)
; Converts a given object to a string
(define (lazy:string object)
  (cond
    ((string? object) object)
    ((nodeset? object) (if (lazy:null? object)
                           ""
                           (lazy:string-value (lazy:car object))))
    ((number? object)
     (if (and (rational? object) (not (integer? object)))  ; like 1/2
         (number->string (exact->inexact object))
         (number->string object)))
    ((boolean? object) (if object "true" "false"))
    (else   ; Unknown type -> empty string.   
     "")))

; The counterpart to XPath 'boolean' function (section 4.3 XPath Rec.)
; Converts its argument to a boolean
(define (lazy:boolean object)
  (cond
    ((boolean? object) object)
    ((number? object) (not (= object 0)))
    ((string? object) (> (string-length object) 0))
    ((nodeset? object) (not (lazy:null? object)))
    (else  ; Not specified in XPath Rec.
     #f)))

; The counterpart to XPath 'number' function (section 4.4 XPath Rec.)
; Converts its argument to a number
; NOTE: 
;  1. The argument is not optional (yet?)
;  2. string->number conversion is not IEEE 754 round-to-nearest
;  3. NaN is represented as 0
(define (lazy:number obj)
  (cond
    ((number? obj) obj)
    ((string? obj)
     (let ((nmb (call-with-input-string obj read)))
       (if (number? nmb)
	 nmb
	 0))) ; NaN
    ((boolean? obj) (if obj 1 0))
    ((nodeset? obj) (lazy:number (lazy:string obj)))
    (else 0))) ; unknown datatype

; Returns a string value for a given node in accordance to
; XPath Rec. 5.1 - 5.7
; Undocumented functionality - can be applied for a node that is a promise
(define (lazy:string-value node)
  (cond
    ((lazy:promise? node)
     (let ((value (force node)))
       (if (nodeset? value)
           (apply string-append
                  (map lazy:string-value value))
           (lazy:string-value value))))
    ((not (pair? node))  ; a text node?
     (if (string? node)
         node ""))
    ((lazy:null? (cdr node))  ; null content
     "")
    (else
     (apply
      string-append
      (cons ""
            (map
             lazy:string-value
             (let ((frst (lazy:car (cdr node))))
               (if
                (and (pair? frst) (eq? '@ (car frst)))  ; attribute node
                (lazy:cdr (cdr node))
                (cdr node)))))))))
                
;-------------------------------------------------
; Comparators for XPath objects 

; A helper for XPath equality operations: = , !=
;  'bool-op', 'number-op' and 'string-op' are comparison operations for 
; a pair of booleans,  numbers and strings respectively
(define (lazy:equality-cmp bool-op number-op string-op)
  (lambda (obj1 obj2)
    (cond
      ((and (not (nodeset? obj1)) (not (nodeset? obj2)))  
       ; neither object is a nodeset
       (cond
         ((boolean? obj1) (bool-op obj1 (sxml:boolean obj2)))
         ((boolean? obj2) (bool-op (sxml:boolean obj1) obj2))
         ((number? obj1) (number-op obj1 (sxml:number obj2)))
         ((number? obj2) (number-op (sxml:number obj1) obj2))
         (else  ; both objects are strings
          (string-op obj1 obj2))))
      ((and (nodeset? obj1) (nodeset? obj2))  ; both objects are nodesets
       (let first ((str-set1 (lazy:map lazy:string-value obj1))
                   (str-set2 (lazy:map lazy:string-value obj2)))
         (cond
           ((null? str-set1) #f)
           ((lazy:promise? (car str-set1))   ; time to get the next portion
            (first (append (as-nodeset (force (car str-set1)))
                           (cdr str-set1))
                   str-set2))
           ((let second ((elem (car str-set1))
                         (set2 str-set2))
              (cond
                ((null? set2) #f)
                ((lazy:promise? (car set2))   ; time to get the next portion
                 (second elem
                         (append (as-nodeset (force (car set2)))
                                 (cdr set2))))
                ((string-op elem (car set2)) #t)
                (else (second elem (cdr set2))))) #t)
           (else
            (first (cdr str-set1) str-set2)))))
      (else  ; one of the objects is a nodeset, the other is not
       (call-with-values
        (lambda ()
          (if (nodeset? obj1) (values obj1 obj2) (values obj2 obj1)))
        (lambda (nset elem)
          (cond
            ((boolean? elem) (bool-op elem (lazy:boolean nset)))
            ((number? elem)
             (let loop ((nset
                         (lazy:map
                          (lambda (node) (lazy:number (lazy:string-value node)))
                          nset)))
               (cond
                 ((null? nset) #f)
                 ((lazy:promise? (car nset))  ; time to get the next portion
                  (loop (append (as-nodeset (force (car nset)))
                                (cdr nset))))
                 ((number-op elem (car nset)) #t)
                 (else (loop (cdr nset))))))
            ((string? elem)
             (let loop ((nset (lazy:map lazy:string-value nset)))
               (cond
                 ((null? nset) #f)
                 ((lazy:promise? (car nset))  ; time to get the next portion
                  (loop (append (as-nodeset (force (car nset)))
                                (cdr nset))))
                 ((string-op elem (car nset)) #t)
                 (else (loop (cdr nset))))))
            (else  ; unknown datatype
             (cerr "Unknown datatype: " elem nl)
             #f))))))))

(define lazy:equal? (lazy:equality-cmp eq? = string=?))

(define lazy:not-equal?
  (lazy:equality-cmp
   (lambda (bool1 bool2) (not (eq? bool1 bool2)))
   (lambda (num1 num2) (not (= num1 num2)))
   (lambda (str1 str2) (not (string=? str1 str2)))))

; Relational operation ( < , > , <= , >= ) for two XPath objects
;  op is comparison procedure: < , > , <= or >=
(define (lazy:relational-cmp op)
  (lambda (obj1 obj2)
    (cond
      ((not (or (nodeset? obj1) (nodeset? obj2)))  ; neither obj is a nodeset
       (op (lazy:number obj1) (lazy:number obj2)))
      ((boolean? obj1)  ; 'obj1' is a boolean, 'obj2' is a nodeset
       (op (lazy:number obj1) (lazy:number (lazy:boolean obj2))))
      ((boolean? obj2)  ; 'obj1' is a nodeset, 'obj2' is a boolean
       (op (lazy:number (lazy:boolean obj1)) (lazy:number obj2)))
      ((or (null? obj1) (null? obj2)) ; one of the objects is an empty nodeset
       #f)
      (else  ; at least one object is a nodeset
       (op
        (cond
          ((nodeset? obj1)  ; 'obj1' is a (non-empty) nodeset
           (let ((nset1 (lazy:map
                         (lambda (node) (lazy:number (lazy:string-value node)))
                         obj1)))
             (let first ((num1 (car nset1))
                         (nset1 (cdr nset1)))
               (cond
                 ((null? nset1) num1)
                 ((lazy:promise? (car nset1))  ; time to obtain the next portion
                  (first num1
                         (apply (as-nodeset (force (car nset1)))
                                (cdr nset1))))
                 ((op num1 (car nset1)) (first num1 (cdr nset1)))
                 (else (first (car nset1) (cdr nset1)))))))
          ((string? obj1) (sxml:number obj1))
          (else  ; 'obj1' is a number
           obj1))
        (cond
          ((nodeset? obj2)  ; 'obj2' is a (non-empty) nodeset
           (let ((nset2 (lazy:map
                         (lambda (node) (lazy:number (lazy:string-value node)))
                         obj2)))
             (let second ((num2 (car nset2))
                          (nset2 (cdr nset2)))
               (cond
                 ((null? nset2) num2)
                 ((lazy:promise? (car nset2))  ; time to obtain the next portion
                  (second num2
                          (apply (as-nodeset (force (car nset2)))
                                 (cdr nset2))))
                 ((op num2 (car nset2)) (second (car nset2) (cdr nset2)))
                 (else (second num2 (cdr nset2)))))))
          ((string? obj2) (sxml:number obj2))
          (else  ; 'obj2' is a number
           obj2)))))))


;==========================================================================
; XPath Core Function Library

;-------------------------------------------------
; 4.1 Node Set Functions

; last()
(define (lazy:core-last num-anc)
  (lambda (nodeset position+size var-binding)
    (cdr position+size)))
  
; position()
(define (lazy:core-position num-anc)
  (lambda (nodeset position+size var-binding)
    (car position+size)))

; count(node-set)
(define (lazy:core-count num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (let ((res (arg-func nodeset position+size var-binding)))
      (cond
        ((nodeset? res) (lazy:length res))
        (else
         (sxml:xpointer-runtime-error
          "count() function - an argument is not a nodeset")
         0)))))

; id(object)
(define (lazy:core-id num-anc arg-func) 
  (lambda (nodeset position+size var-binding)    
    (let* ((root-node (list (lazy:car
                             (lazy:reach-root nodeset))))
           (id-nset ((sxml:child (ntype?? 'id-index))
                     ((sxml:child (ntype?? '@@)) root-node))))
      (if
       (null? id-nset)  ; no id-index
       '()  ; ID function returns an empty nodeset
       (let ((res ((sxml:id (cdar id-nset))  ; implemented in "sxpath-ext.scm"
                   (lazy:result->list
                    (lazy:contextset->nodeset
                     (arg-func nodeset position+size var-binding))))))
         (if (and num-anc (zero? num-anc))  ; no ancestors required
             res
             (lazy:recover-contextset res root-node num-anc)))))))

; local-name(node-set?)
(define (lazy:core-local-name num-anc . arg-func)  ; optional argument 
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (let ((nodeset (lazy:contextset->nodeset nodeset)))
          (cond
            ((lazy:null? nodeset) "")
            ((not (pair? (lazy:car nodeset))) "")  ; no name
            (else
             (let ((name (symbol->string (car (lazy:car nodeset)))))
               (cond
                 ((string-rindex name #\:)
                  => (lambda (pos)
                       (substring name (+ pos 1) (string-length name))))
                 (else  ; a NCName
                  name)))))))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)          
          (let ((obj
                 (lazy:contextset->nodeset
                  (func nodeset position+size var-binding))))
            (cond
              ((null? obj) "")  ; an empty nodeset
              ((not (nodeset? obj))
               (sxml:xpointer-runtime-error
                "NAME function - an argument is not a nodeset")              
               "")
              ((not (pair? (lazy:car obj))) "")  ; no name
              (else
               (let ((name (symbol->string (car (lazy:car obj)))))
                 (cond
                   ((string-rindex name #\:)
                    => (lambda (pos)
                         (substring
                          name (+ pos 1) (string-length name))))
                   (else  ; a NCName
                    name))))))))))

; namespace-uri(node-set?)
(define (lazy:core-namespace-uri num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (let ((nodeset (lazy:contextset->nodeset nodeset)))
          (cond
            ((lazy:null? nodeset) "")
            ((not (pair? (lazy:car nodeset))) "")  ; no name
            (else
             (let ((name (symbol->string (car (lazy:car nodeset)))))
               (cond
                 ((string-rindex name #\:)
                  => (lambda (pos)
                       (substring name 0 pos)))
                 (else  ; a NCName
                  "")))))))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)          
          (let ((obj
                 (lazy:contextset->nodeset
                  (func nodeset position+size var-binding))))           
            (cond
              ((lazy:null? obj) "")  ; an empty nodeset
              ((not (nodeset? obj))
               (sxml:xpointer-runtime-error
                "NAME function - an argument is not a nodeset")
               "")
              ((not (pair? (lazy:car obj))) "")  ; no name
              (else
               (let ((name (symbol->string (car (lazy:car obj)))))
                 (cond
                   ((string-rindex name #\:)
                    => (lambda (pos)
                         (substring name 0 pos)))
                   (else ""))))))))))

; name(node-set?)
(define (lazy:core-name num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (let ((nodeset (lazy:contextset->nodeset nodeset)))
          (cond
            ((lazy:null? nodeset) "")
            ((not (pair? (lazy:car nodeset))) "")  ; no name
            (else
             (symbol->string (car (lazy:car nodeset)))))))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)
          (let ((obj
                 (lazy:contextset->nodeset
                  (func nodeset position+size var-binding))))        
            (cond
              ((lazy:null? obj) "")  ; an empty nodeset
              ((not (nodeset? obj))
               (sxml:xpointer-runtime-error
                "NAME function - an argument is not a nodeset")
               "")
              ((not (pair? (lazy:car obj))) "")  ; no name
              (else
               (symbol->string (car (lazy:car obj))))))))))

;-------------------------------------------------
; 4.2 String Functions

; string(object?)
(define (lazy:core-string num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (lazy:string
         (lazy:contextset->nodeset nodeset)))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)
          (lazy:string
           (lazy:contextset->nodeset
            (func nodeset position+size var-binding)))))))

; concat(string, string, string*)
(define (lazy:core-concat num-anc . arg-func-lst)
  (lambda (nodeset position+size var-binding)
    (apply
     string-append
     (map
      (lambda (f)
        (lazy:string
         (lazy:contextset->nodeset
          (f nodeset position+size var-binding))))
      arg-func-lst))))

; starts-with(string, string)
(define (lazy:core-starts-with num-anc arg-func1 arg-func2)
  (lambda (nodeset position+size var-binding)
    (let ((str1 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func1 nodeset position+size var-binding))))
          (str2 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func2 nodeset position+size var-binding)))))
      (string-prefix? str2 str1))))

; contains(string, string)
(define (lazy:core-contains num-anc arg-func1 arg-func2)
  (lambda (nodeset position+size var-binding)
    (let ((str1 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func1 nodeset position+size var-binding))))
          (str2 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func2 nodeset position+size var-binding)))))
      (if (substring? str2 str1) #t #f)  ; must return a boolean
      )))

; substring-before(string, string)
(define (lazy:core-substring-before num-anc arg-func1 arg-func2)
  (lambda (nodeset position+size var-binding)
    (let* ((str1 (lazy:string
                  (lazy:contextset->nodeset
                   (arg-func1 nodeset position+size var-binding))))
           (str2 (lazy:string
                  (lazy:contextset->nodeset
                   (arg-func2 nodeset position+size var-binding))))
           (pos (substring? str2 str1)))
      (if (not pos)  ; STR1 doesn't contain STR2
          ""
          (substring str1 0 pos)))))

; substring-after(string, string)
(define (lazy:core-substring-after num-anc arg-func1 arg-func2)
  (lambda (nodeset position+size var-binding)
    (let* ((str1 (lazy:string
                  (lazy:contextset->nodeset
                   (arg-func1 nodeset position+size var-binding))))
           (str2 (lazy:string
                  (lazy:contextset->nodeset
                   (arg-func2 nodeset position+size var-binding))))
           (pos (substring? str2 str1)))
      (if
       (not pos)  ; STR1 doesn't contain STR2
       ""
       (substring
        str1 (+ pos (string-length str2)) (string-length str1))))))

; substring(string, number, number?)
(define (lazy:core-substring num-anc arg-func1 arg-func2 . arg-func3)
  (if (null? arg-func3)  ; no third argument supplied
      (lambda (nodeset position+size var-binding)
        (let ((str (lazy:string
                    (lazy:contextset->nodeset
                     (arg-func1 nodeset position+size var-binding))))
              (num1 (lazy:number
                     (lazy:contextset->nodeset
                      (arg-func2 nodeset position+size var-binding)))))
          (let ((len (string-length str))
                (start (- (inexact->exact (round num1)) 1)))
            (if (> start len)
                ""
                (substring str (if (< start 0) 0 start) len)))))
      (let ((arg-func3 (car arg-func3)))
        (lambda (nodeset position+size var-binding)
          (let ((str (lazy:string
                      (lazy:contextset->nodeset
                       (arg-func1 nodeset position+size var-binding))))
                (num1 (lazy:number
                       (lazy:contextset->nodeset
                        (arg-func2 nodeset position+size var-binding))))
                (num2 (lazy:number
                       (lazy:contextset->nodeset
                        (arg-func3 nodeset position+size var-binding)))))
            (let* ((len (string-length str))
                   (start (- (inexact->exact (round num1)) 1))
                   (fin (+ start (inexact->exact (round num2)))))
              (if (or (> start len) (< fin 0) (< fin start))
                  ""
                  (substring str
                             (if (< start 0) 0 start)
                             (if (> fin len) len fin)))))))))

; string-length(string?)
(define (lazy:core-string-length num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (string-length
         (lazy:string (lazy:contextset->nodeset nodeset))))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)
          (string-length
           (lazy:string
            (lazy:contextset->nodeset
             (func nodeset position+size var-binding))))))))

; normalize-space(string?)
(define (lazy:core-normalize-space num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (let rpt ((src (string-split
                        (lazy:string (lazy:contextset->nodeset nodeset))
                        sxml:whitespace))
                  (res '()))
          (cond
            ((null? src)
             (apply string-append (reverse res)))
            ((= (string-length (car src)) 0)  ; empty string
             (rpt (cdr src) res))
            ((null? res)
             (rpt (cdr src) (cons (car src) res)))
            (else
             (rpt (cdr src) (cons (car src) (cons " " res)))))))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)
          (let rpt ((src (string-split
                          (lazy:string
                           (lazy:contextset->nodeset
                            (func nodeset position+size var-binding)))
                          sxml:whitespace))
                    (res '()))
            (cond
              ((null? src)
               (apply string-append (reverse res)))
              ((= (string-length (car src)) 0)  ; empty string
               (rpt (cdr src) res))
              ((null? res)
               (rpt (cdr src) (cons (car src) res)))
              (else
               (rpt (cdr src) (cons (car src) (cons " " res))))))))))

; translate(string, string, string)
(define (lazy:core-translate num-anc arg-func1 arg-func2 arg-func3)
  (lambda (nodeset position+size var-binding)    
    (let ((str1 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func1 nodeset position+size var-binding))))
          (str2 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func2 nodeset position+size var-binding))))
          (str3 (lazy:string
                 (lazy:contextset->nodeset
                  (arg-func3 nodeset position+size var-binding)))))
      (let ((alist
             (let while ((lst2 (string->list str2))
                         (lst3 (string->list str3))
                         (alist '()))
               (cond
                 ((null? lst2) (reverse alist))
                 ((null? lst3)
                  (append
                   (reverse alist)
                   (map
                    (lambda (ch) (cons ch #f))
                    lst2)))
                 (else
                  (while
                   (cdr lst2)
                   (cdr lst3)
                   (cons (cons (car lst2) (car lst3)) alist)))))))
        (let rpt ((lst1 (string->list str1))
                  (res '()))
          (cond
            ((null? lst1) (list->string (reverse res)))
            ((assoc (car lst1) alist)
             => (lambda (pair)
                  (if (cdr pair)
                      (rpt (cdr lst1) (cons (cdr pair) res))
                      (rpt (cdr lst1) res))))
            (else
             (rpt (cdr lst1) (cons (car lst1) res)))))))))

;-------------------------------------------------
; 4.3 Boolean Functions

; boolean(object)
(define (lazy:core-boolean num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (lazy:boolean
     (arg-func nodeset position+size var-binding))))

; not(boolean)
(define (lazy:core-not num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (not (lazy:boolean 
          (arg-func nodeset position+size var-binding)))))

; true()
(define (lazy:core-true num-anc)
  (lambda (nodeset position+size var-binding) #t))

; false()
(define (lazy:core-false num-anc)
  (lambda (nodeset position+size var-binding) #f))

; lang(string)
(define (lazy:core-lang num-anc arg-func)
  (lambda (nodeset position+size var-binding)    
    (let ((arg (lazy:string
                (lazy:contextset->nodeset
                 (arg-func nodeset position+size var-binding))))
          (lng
           ((lazy:child (ntype?? '*text*))
            ((lazy:attribute (ntype?? 'xml:lang))
             ((lazy:ancestor-or-self (lambda (x) #t))
              (lazy:car nodeset)  ; context-node = (car nodeset)
              )))))
      (and (not (null? lng))
           (or (string-ci=? arg (lazy:car lng))
               (string-prefix-ci? (string-append arg "-") (lazy:car lng)))))))

;-------------------------------------------------
; 4.4 Number Functions

; number(object?)
(define (lazy:core-number num-anc . arg-func)  ; optional argument
  (if (null? arg-func)  ; no argument supplied
      (lambda (nodeset position+size var-binding)
        (lazy:number (lazy:contextset->nodeset nodeset)))
      (let ((func (car arg-func)))
        (lambda (nodeset position+size var-binding)
          (lazy:number
           (lazy:contextset->nodeset
            (func nodeset position+size var-binding)))))))

; sum(node-set)
(define (lazy:core-sum num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (let ((res (arg-func nodeset position+size var-binding)))
      (cond
        ((nodeset? res)
         (apply +
                (map
                 (lambda (node)
                   (lazy:number
                    (lazy:string-value (sxml:context->node node))))
                 (lazy:result->list res))))
        (else
         (sxml:xpointer-runtime-error
          "SUM function - an argument is not a nodeset")
         0)))))

; floor(number)
(define (lazy:core-floor num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (inexact->exact
     (floor (lazy:number
             (lazy:contextset->nodeset
              (arg-func nodeset position+size var-binding)))))))

; ceiling(number)
(define (lazy:core-ceiling num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (inexact->exact
     (ceiling (lazy:number
               (lazy:contextset->nodeset
                (arg-func nodeset position+size var-binding)))))))

; round(number)
(define (lazy:core-round num-anc arg-func)
  (lambda (nodeset position+size var-binding)
    (inexact->exact
     (round (lazy:number
             (lazy:contextset->nodeset
              (arg-func nodeset position+size var-binding)))))))


;=========================================================================
; XPath AST processing
; AST is considered to be properly formed
       
; {5} <AxisSpecifier> ::= (axis-specifier  <AxisName> )
; {6} <AxisName> ::= (ancestor)
;                    | (ancestor-or-self)
;                    | (attribute)
;                    | (child)
;                    | (descendant)
;                    | (descendant-or-self)
;                    | (following)
;                    | (following-sibling)
;                    | (namespace)
;                    | (parent)
;                    | (preceding)
;                    | (preceding-sibling)
;                    | (self)
;                    | (arc)  ; the following 3 are added by SXLink
;                    | (traverse)
;                    | (traverse-arc)
; Returns:  (cons lambda num-ancestors)
(define (lazy:ast-axis-specifier op num-anc)
  (if
   (not (eq? (car op) 'axis-specifier))
   (draft:signal-semantic-error "not an AxisSpecifier - " op)
   (case (caadr op)  ; AxisName
     ((ancestor)
      (cons lazy:ancestor #f))
     ((ancestor-or-self)
      (cons lazy:ancestor-or-self #f))
     ((attribute)
      (cons lazy:attribute (draft:na-minus-nneg num-anc 1)))
     ((child)
      (cons lazy:child (draft:na-minus-nneg num-anc 1)))
     ((descendant)
      (cons lazy:descendant (draft:na-minus-nneg num-anc 1)))
     ((descendant-or-self)
      (cons lazy:descendant-or-self num-anc))
     ((following)
      (cons lazy:following #f))
     ((following-sibling)
      (cons lazy:following-sibling (draft:na-max num-anc 1)))
     ((namespace)
      (cons lazy:namespace (draft:na-minus-nneg num-anc 1)))
     ((parent)
      (cons lazy:parent (draft:na+ num-anc 1)))
     ((preceding)
      (cons lazy:preceding #f))
     ((preceding-sibling)
      (cons lazy:preceding-sibling (draft:na-max num-anc 1)))
     ((self)
      (cons lazy:self num-anc))     
     (else
      (draft:signal-semantic-error "unknown AxisName - " op)))))

;-------------------------------------------------
; In this section, each function accepts 2 arguments
;  op - S-expression which represents the operation
;  num-anc - how many ancestors are required in the context after that
;            operation
; and returns either #f, which signals of a semantic error, or
;  (list (lambda (nodeset position+size var-binding) ...)
;        num-anc-it-requires
;        requires-size? )
;  position+size - the same to what was called 'context' in TXPath-1
;  requires-size? - context size in required for evaluating the operation

; {1} <LocationPath> ::= <RelativeLocationPath>
;                        | <AbsoluteLocationPath>
(define (lazy:ast-location-path op num-anc)
  (case (car op)
    ((absolute-location-path)
     (lazy:ast-absolute-location-path op num-anc))
    ((relative-location-path)
     (lazy:ast-relative-location-path op num-anc))
    (else
     (draft:signal-semantic-error "improper LocationPath - " op))))

; {2} <AbsoluteLocationPath> ::= (absolute-location-path  <Step>* )
(define (lazy:ast-absolute-location-path op num-anc)
  (cond
    ((not (eq? (car op) 'absolute-location-path))
     (draft:signal-semantic-error "not an AbsoluteLocationPath - " op))
    ((null? (cdr op))  ; no Steps
     (list
      (lambda (nodeset position+size var-binding)
        (lazy:reach-root nodeset))
      #f  ; num-ancestors
      #f  ; requires-size?
      ))
    (else
     (and-let*
      ((steps-res (lazy:ast-step-list (cdr op) num-anc)))
      (list
       (if
        (null? (cdar steps-res))  ; only a single step
        (let ((step-impl (caar steps-res)))
          (lambda (nodeset position+size var-binding)
            (step-impl
             (lazy:reach-root nodeset) position+size var-binding)))
        (let ((converters (car steps-res)))
          (lambda (nodeset position+size var-binding)
            (let rpt ((nset (lazy:reach-root nodeset))
                      (fs converters))
              (if (null? fs)
                  nset
                  (rpt ((car fs) nset position+size var-binding)
                       (cdr fs)))))))
       #f  ; num-ancestors
       #f  ; requires-size?
       )))))

; {3} <RelativeLocationPath> ::= (relative-location-path  <Step>+ )
(define (lazy:ast-relative-location-path op num-anc)
  (if
   (not (eq? (car op) 'relative-location-path))
   (draft:signal-semantic-error "not a RelativeLocationPath - " op)
   (and-let*
    ((steps-res (lazy:ast-step-list (cdr op) num-anc)))
    (list
     (if
      (null? (cdar steps-res))  ; only a single step
      (caar steps-res)
      (let ((converters (car steps-res)))
        (lambda (nodeset position+size var-binding)
          (let rpt ((nset nodeset)
                    (fs converters))
            (if (null? fs)
                nset
                (rpt ((car fs) nset position+size var-binding)
                     (cdr fs)))))))
     (cadr steps-res)  ; num-ancestors
     #f  ; requires-size?
     ))))

; {4} <Step> ::= (step  <AxisSpecifier> <NodeTest> <Predicate>* )
;                | (range-to  (expr <Expr>)  <Predicate>* )
(define (lazy:ast-step op num-anc)
  (cond
    ((eq? (car op) 'range-to)
     (draft:signal-semantic-error "range-to function not implemented"))
    ((eq? (car op) 'filter-expr)
     (lazy:ast-filter-expr op num-anc))
    ((eq? (car op) 'lambda-step)  ; created by sxpath
     (let ((proc (cadr op)))
       (list
        (if
         (and num-anc (zero? num-anc))  ; no ancestors required
         (lambda (nodeset position+size var-binding)
           (proc (lazy:contextset->nodeset (as-nodeset nodeset))
                 var-binding))
         (lambda (nodeset position+size var-binding)
           (lazy:find-proper-context
            (proc (lazy:contextset->nodeset (as-nodeset nodeset))
                  var-binding)
            (as-nodeset nodeset)
            num-anc)))
        num-anc  ; num-ancestors
        #f  ; requires-last?
        )))
    ((eq? (car op) 'step)
     (if
      (null? (cdddr op))  ; no Predicates
      (and-let*
       ((axis-lst (lazy:ast-axis-specifier (cadr op) num-anc))
        (ntest (draft:ast-node-test (caddr op))))
       (let ((axis
              (lazy:axis-consume-nodeset
               ((car axis-lst) ntest num-anc))))
         (list
          (lambda (nodeset position+size var-binding)
            (axis nodeset))
          (cdr axis-lst)  ; num-ancestors
          #f  ; requires-size?
          )))
      (and-let*
       ((preds-res (lazy:ast-predicate-list (cdddr op) 0))
        (axis-lst (lazy:ast-axis-specifier
                   (cadr op) (draft:na-max num-anc (cadr preds-res))))
        (ntest (draft:ast-node-test (caddr op))))
       (let ((axis ((car axis-lst)
                    ntest (draft:na-max num-anc (cadr preds-res))))
             (pred-impl-lst (car preds-res)))
         (list
          (lambda (nodeset position+size var-binding)
            (let iter-src ((src nodeset)
                           (candidates '())
                           (res '()))
              (cond
                ((null? candidates)  ; consume the following node from src
                 (cond
                   ((null? src)  ; iteration is over
                    (reverse res))
                   ((lazy:promise? (car src))
                    (if
                     (null? res)  ; result is still empty, need to force src
                     (iter-src (append (as-nodeset (force (car src)))
                                       (cdr src))
                               candidates
                               res)
                     (reverse  ; otherwise - return the result with a promise
                      (cons
                       (delay (iter-src src candidates '()))
                       res))))
                   (else  ; (car src) is a node
                    (iter-src
                     (cdr src)
                     (let iter-preds ((nset (axis (car src)))
                                      (preds pred-impl-lst))
                       (if
                        (null? preds)
                        nset
                        (iter-preds
                         ((car preds) nset position+size var-binding)
                         (cdr preds))))
                     res))))
                ((lazy:promise? (car candidates))
                 ; First candidate is a promise
                 (if
                  (null? res)  ; result is still empty, need to force candidate
                  (iter-src src
                            (append (as-nodeset (force (car candidates)))
                                    (cdr candidates))
                            res)
                  (reverse  ; otherwise - return the result with a promise
                   (cons
                    (delay (iter-src src candidates '()))
                    res))))
                (else   ; the first candidate is a node
                 (iter-src src (cdr candidates)
                           (cons (car candidates) res))))))
          (cdr axis-lst)  ; num-ancestors
          #f  ; requires-last?
         )))))
    (else
     (draft:signal-semantic-error "not a Step - " op))))

; {4a} ( <Step>+ )
; Returns (list (listof step-impl) num-anc) or #f
; NOTE: requires-size? is not needed here, since it is always #f
(define (lazy:ast-step-list step-lst num-anc)
  (let loop ((steps-to-view (reverse step-lst))
             (res-lst '())
             (num-anc num-anc))
    (if
     (null? steps-to-view)  ; everyone processed
     (list res-lst num-anc)
     (and-let*
      ((step-res (lazy:ast-step (car steps-to-view) num-anc)))
      (loop
       (cdr steps-to-view)
       (cons (car step-res) res-lst)
       (cadr step-res))))))

; {8} <Predicate> ::= (predicate  <Expr> )
; NOTE: num-anc is dummy here, since it is always 0 for Predicates
(define (lazy:ast-predicate op num-anc)
  (if
   (not (eq? (car op) 'predicate))
   (draft:signal-semantic-error "not an Predicate - " op)
   (and-let*
    ((expr-res (lazy:ast-expr (cadr op) 0)))
    (let ((pred (car expr-res)))
      (list
       (if
        (caddr expr-res)  ; requires-last?
        (lambda (nodeset position+size var-binding)
          (if
           (null? nodeset)  ; already empty
           nodeset  ; nothing to filter
           (let ((size (lazy:length nodeset)))
             (let loop ((nset nodeset)
                        (res '())
                        (pos 1))
               (cond
                 ((null? nset)
                  (reverse res))
                 ((lazy:promise? (car nset))
                  ; This promise was already forced when evaluating lazy:length
                  (loop (append (as-nodeset (force (car nset)))
                                (cdr nset))
                        res pos))
                 (else  ; (car nset) is a node
                  (let ((value (pred (list (car nset))
                                     (cons pos size)
                                     var-binding)))
                    (loop (cdr nset)
                          (if (if (number? value)
                                  (= value pos)
                                  (lazy:boolean value))
                              (cons (car nset) res)
                              res)
                          (+ pos 1)))))))))
        (lambda (nodeset position+size var-binding)
          (if
           (null? nodeset)  ; already empty
           nodeset  ; nothing to filter
           (let loop ((nset nodeset)
                      (res '())
                      (pos 1))
             (cond
               ((null? nset)
                (reverse res))
               ((lazy:promise? (car nset))
                (reverse
                 (cons
                  (delay (loop
                          (append (as-nodeset (force (car nset)))
                                  (cdr nset))
                          '()  ; turns res to empty
                          pos))
                  res)))
               (else  ; (car nset) is a node
                (let ((value (pred (list (car nset))
                                   (cons pos 1)  ; context size is dummy
                                   var-binding)))
                  (loop (cdr nset)
                        (if (if (number? value)
                                (= value pos)
                                (lazy:boolean value))
                            (cons (car nset) res)
                            res)
                        (+ pos 1)))))))))
       (cadr expr-res)  ; num-ancestors
       (caddr expr-res)  ; requires-last?
       )))))

; {8a} ( <Predicate>+ )
; Returns (list (listof pred-impl) num-anc) or #f
; NOTE: num-anc is dummy here, since it is always 0 for Predicates
(define (lazy:ast-predicate-list op-lst num-anc)
  (let ((pred-res-lst
         (map
          (lambda (op) (lazy:ast-predicate op 0))
          op-lst)))
    (if
     (member #f pred-res-lst)  ; error detected
     #f
     (list
      (map car pred-res-lst)
      (apply draft:na-max (map cadr pred-res-lst))))))

; {9} <Expr> ::= <OrExpr>
;                | <AndExpr>
;                | <EqualityExpr>
;                | <RelationalExpr>
;                | <AdditiveExpr>
;                | <MultiplicativeExpr>
;                | <UnionExpr>
;                | <PathExpr>
;                | <FilterExpr>
;                | <VariableReference>
;                | <Literal>
;                | <Number>
;                | <FunctionCall>
;                | <LocationPath>
(define (lazy:ast-expr op num-anc)
  (case (car op)
    ((or)
     (lazy:ast-or-expr op num-anc))
    ((and)
     (lazy:ast-and-expr op num-anc))
    ((= !=)
     (lazy:ast-equality-expr op num-anc))
    ((< > <= >=)
     (lazy:ast-relational-expr op num-anc))
    ((+ -)
     (lazy:ast-additive-expr op num-anc))
    ((* div mod)
     (lazy:ast-multiplicative-expr op num-anc))
    ((union-expr)
     (lazy:ast-union-expr op num-anc))
    ((path-expr)
     (lazy:ast-path-expr op num-anc))
    ((filter-expr)
     (lazy:ast-filter-expr op num-anc))
    ((variable-reference)
     (lazy:ast-variable-reference op num-anc))
    ((literal)
     (lazy:ast-literal op num-anc))
    ((number)
     (lazy:ast-number op num-anc))
    ((function-call)
     (lazy:ast-function-call op num-anc))
    ((absolute-location-path)
     (lazy:ast-absolute-location-path op num-anc))
    ((relative-location-path)
     (lazy:ast-relative-location-path op num-anc))
    (else
     (draft:signal-semantic-error "unknown Expr - " op))))

; {10} <OrExpr> ::= (or <Expr> <Expr>+ )
; NOTE: num-anc is dummy here, since it is always 0 for OrExpr
(define (lazy:ast-or-expr op num-anc)
  (let ((expr-res-lst
         (map
          (lambda (expr) (lazy:ast-expr expr 0))
          (cdr op))))
    (if
     (member #f expr-res-lst)  ; error detected
     #f
     (let ((expr-impls (map car expr-res-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (let rpt ((fs expr-impls))
          (cond
            ((null? fs) #f)
            ((lazy:boolean ((car fs) nodeset position+size var-binding)) #t)
            (else (rpt (cdr fs))))))
      (apply draft:na-max (map cadr expr-res-lst))  ; num-ancestors
      (apply lazy:or (map caddr expr-res-lst))  ; requires-last?
      )))))

; {11} <AndExpr> ::= (and <Expr> <Expr>+ )
; NOTE: num-anc is dummy here, since it is always 0 for AndExpr
(define (lazy:ast-and-expr op num-anc)
  (let ((expr-res-lst
         (map
          (lambda (expr) (lazy:ast-expr expr 0))
          (cdr op))))
    (if
     (member #f expr-res-lst)  ; error detected
     #f
     (let ((expr-impls (map car expr-res-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (let rpt ((fs expr-impls))
          (cond
            ((null? fs) #t)
            ((not
              (lazy:boolean ((car fs) nodeset position+size var-binding)))
             #f)
            (else (rpt (cdr fs))))))
      (apply draft:na-max (map cadr expr-res-lst))  ; num-ancestors
      (apply lazy:or (map caddr expr-res-lst))  ; requires-last?
      )))))

; {12} <EqualityExpr> ::= (=  <Expr> <Expr> )
;                         | (!=  <Expr> <Expr> )
; NOTE: num-anc is dummy here, since it is always 0 for EqualityExpr
(define (lazy:ast-equality-expr op num-anc)
  (and-let*
   ((left-lst (lazy:ast-expr (cadr op) 0))
    (right-lst (lazy:ast-expr (caddr op) 0)))
   (let ((cmp-op (cadr (assq (car op) `((= ,lazy:equal?)
                                        (!= ,lazy:not-equal?)))))
         (left (car left-lst))
         (right (car right-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (cmp-op
         (lazy:contextset->nodeset
          (left nodeset position+size var-binding))
         (lazy:contextset->nodeset
          (right nodeset position+size var-binding))))
      (draft:na-max (cadr left-lst) (cadr right-lst))  ; num-ancestors
      (or (caddr left-lst) (caddr right-lst))  ; requires-last?
      ))))

; {13} <RelationalExpr> ::= (<  <Expr> <Expr> )
;                           | (>  <Expr> <Expr> )
;                           | (<=  <Expr> <Expr> )
;                           | (>=  <Expr> <Expr> )
; NOTE: num-anc is dummy here, since it is always 0 for RelationalExpr
(define (lazy:ast-relational-expr op num-anc)
  (and-let*
   ((left-lst (lazy:ast-expr (cadr op) 0))
    (right-lst (lazy:ast-expr (caddr op) 0)))
   (let ((cmp-op
          (lazy:relational-cmp
           (cadr (assq (car op) `((< ,<) (> ,>) (<= ,<=) (>= ,>=))))))
         (left (car left-lst))
         (right (car right-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (cmp-op
         (lazy:contextset->nodeset
          (left nodeset position+size var-binding))
         (lazy:contextset->nodeset
          (right nodeset position+size var-binding))))
      (draft:na-max (cadr left-lst) (cadr right-lst))  ; num-ancestors
      (or (caddr left-lst) (caddr right-lst))  ; requires-last?
      ))))

; {14} <AdditiveExpr> ::= (+  <Expr> <Expr> )
;                         | (-  <Expr> <Expr>? )
; NOTE: num-anc is dummy here, since it is always 0 for AdditiveExpr
(define (lazy:ast-additive-expr op num-anc)
  (let ((expr-res-lst
         (map
          (lambda (expr) (lazy:ast-expr expr 0))
          (cdr op))))
    (if
     (member #f expr-res-lst)  ; error detected
     #f
     (let ((add-op (cadr (assq (car op) `((+ ,+) (- ,-)))))
           (expr-impls (map car expr-res-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (apply
         add-op
         (map
          (lambda (expr)
            (lazy:number
             (lazy:contextset->nodeset
              (expr nodeset position+size var-binding))))
          expr-impls)))
      (apply draft:na-max (map cadr expr-res-lst))  ; num-ancestors
      (apply lazy:or (map caddr expr-res-lst))  ; requires-last?
      )))))

; {15} <MultiplicativeExpr> ::= (*  <Expr> <Expr> )
;                               | (div  <Expr> <Expr> )
;                               | (mod  <Expr> <Expr> )
; NOTE: num-anc is dummy here, since it is always 0 for MultiplicativeExpr
(define (lazy:ast-multiplicative-expr op num-anc)
  (and-let*
   ((left-lst (lazy:ast-expr (cadr op) 0))
    (right-lst (lazy:ast-expr (caddr op) 0)))
   (let ((mul-op
          (cadr (assq (car op) `((* ,*) (div ,/) (mod ,remainder)))))
         (left (car left-lst))
         (right (car right-lst)))
     (list
      (lambda (nodeset position+size var-binding)
        (mul-op
         (lazy:number
          (lazy:contextset->nodeset
           (left nodeset position+size var-binding)))
         (lazy:number
          (lazy:contextset->nodeset
           (right nodeset position+size var-binding)))))
      (draft:na-max (cadr left-lst) (cadr right-lst))  ; num-ancestors
      (or (caddr left-lst) (caddr right-lst))  ; requires-last?
      ))))

; {16} <UnionExpr> ::= (union-expr  <Expr> <Expr>+ )
(define (lazy:ast-union-expr op num-anc)
  (let ((expr-res-lst
         (map
          (lambda (expr) (lazy:ast-expr expr 0))
          (cdr op))))
    (if
     (member #f expr-res-lst)  ; error detected
     #f
     (let ((expr-impls (map car expr-res-lst)))
       (list
        (lambda (nodeset position+size var-binding)
          (let iter-operands ((fs expr-impls)
                              (candidates '())
                              (res '()))
            (cond
              ((null? candidates)
               (if
                (null? fs)  ; no more operands to be unioned
                (reverse res)                
                (iter-operands
                 (cdr fs)
                 (let ((nset ((car fs) nodeset position+size var-binding)))
                   (cond
                     ((not (nodeset? nset))
                      (sxml:xpointer-runtime-error 
                       "expected - nodeset instead of " nset)
                      '())
                   (else nset)))
                 res)))
              ((lazy:promise? (car candidates))
               (if
                (null? res)  ; res is still null, need to force candidate
                (iter-operands
                 fs
                 (append (as-nodeset (force (car candidates)))
                         (cdr candidates))
                 res)
                (reverse
                 (cons
                  (delay (iter-operands
                          fs
                          (append (as-nodeset (force (car candidates)))
                                  (cdr candidates))
                          '()))
                  res))))
              (else  ; first candidate is a node
               (iter-operands
                fs (cdr candidates) (cons (car candidates) res))))))
        (apply draft:na-max (map cadr expr-res-lst))
        (apply lazy:or (map caddr expr-res-lst))  ; requires-last?
        )))))

; {17} <PathExpr> ::= (path-expr  <FilterExpr> <Step>+ )
(define (lazy:ast-path-expr op num-anc)
  (and-let*
    ((steps-res (lazy:ast-step-list (cddr op) num-anc))
     (filter-lst (lazy:ast-filter-expr (cadr op) (cadr steps-res))))
    (let ((init-impl (car filter-lst))
          (converters (car steps-res)))
      (list
        (lambda (nodeset position+size var-binding)
          (let ((nset
                 (init-impl nodeset position+size var-binding)))
            (let rpt ((nset 
                       (cond
                         ((nodeset? nset) nset)
                         (else
                          (sxml:xpointer-runtime-error 
                           "expected - nodeset instead of " nset)
                          '())))
                      (fs converters))
              (if (null? fs)
                  nset
                  (rpt ((car fs) nset position+size var-binding)
                       (cdr fs))))))
        (cadr filter-lst)  ; num-ancestors
        (caddr filter-lst)  ; requires-last?
        ))))

; {18} <FilterExpr> ::= (filter-expr (primary-expr  <Expr> )
;                                    <Predicate>* )
(define (lazy:ast-filter-expr op num-anc)
  (cond
    ((not (eq? (car op) 'filter-expr))
     (draft:signal-semantic-error "not an FilterExpr - " op))
    ((not (eq? (caadr op) 'primary-expr))
     (draft:signal-semantic-error "not an PrimaryExpr - " (cadr op)))
    ((null? (cddr op))  ; no Predicates
     (lazy:ast-expr (cadadr op) num-anc))
    (else  ; there are predicates
     (and-let*
       ((preds-res (lazy:ast-predicate-list (cddr op) 0))
        (expr-lst (lazy:ast-expr
                   (cadadr op) (draft:na-max num-anc (cadr preds-res)))))
       (let ((expr-impl (car expr-lst))
             (pred-impl-lst (car preds-res)))
         (list
          (lambda (nodeset position+size var-binding)
            (let ((prim-res (expr-impl nodeset position+size var-binding)))
              (let iter-preds ((nset
                                (if
                                 (nodeset? prim-res)
                                 prim-res
                                 (begin 
                                   (sxml:xpointer-runtime-error 
                                    "expected - nodeset instead of " prim-res)
                                   '())))
                               (preds pred-impl-lst))
                (if
                 (null? preds)
                 nset
                 (iter-preds
                  ((car preds) nset position+size var-binding)
                  (cdr preds))))))
          (cadr expr-lst)  ; num-ancestors
          #f  ; requires-last?
         ))))))

; {19} <VariableReference> ::= (variable-reference  <String> )
(define (lazy:ast-variable-reference op num-anc)
  (let ((name (string->symbol (cadr op))))
    (list
     (lambda (nodeset position+size var-binding)
       (cond
         ((assoc name var-binding)
          => cdr)
         (else
          (sxml:xpointer-runtime-error "unbound variable - " name)
          '())))
     0  ; num-ancestors
     #f  ; requires-last?
     )))

; {20} <Literal> ::= (literal  <String> )
(define (lazy:ast-literal op num-anc)
  (let ((literal (cadr op)))
    (list
     (lambda (nodeset position+size var-binding) literal)
     0 #f)))
     
; {21} <Number> :: (number  <Number> )
(define (lazy:ast-number op num-anc)
  (let ((number (cadr op)))
    (list
     (lambda (nodeset position+size var-binding) number)
     0 #f)))

; {22} <FunctionCall> ::= (function-call (function-name  <String> )
;                                        (argument  <Expr> )* )
(define (lazy:ast-function-call op num-anc)
  (let ((core-alist
         ; (list fun-name min-num-args max-num-args na4res impl requires-last?)
         `((last 0 0 0 ,lazy:core-last #t)
           (position 0 0 0 ,lazy:core-position #f)
           (count 1 1 0 ,lazy:core-count #f)
           (id 1 1 #f ,lazy:core-id #f)
           (local-name 0 1 0 ,lazy:core-local-name #f)
           (namespace-uri 0 1 0 ,lazy:core-namespace-uri #f)
           (name 0 1 0 ,lazy:core-name #f)
           (string 0 1 0 ,lazy:core-string #f)
           (concat 2 -1 0 ,lazy:core-concat #f)
           (starts-with 2 2 0 ,lazy:core-starts-with #f)
           (contains 2 2 0 ,lazy:core-contains #f)
           (substring-before 2 2 0 ,lazy:core-substring-before #f)
           (substring-after 2 2 0 ,lazy:core-substring-after #f)
           (substring 2 3 0 ,lazy:core-substring #f)
           (string-length 0 1 0 ,lazy:core-string-length #f)
           (normalize-space 0 1 0 ,lazy:core-normalize-space #f)
           (translate 3 3 0 ,lazy:core-translate #f)
           (boolean 1 1 0 ,lazy:core-boolean #f)
           (not 1 1 0 ,lazy:core-not #f)
           (true 0 0 0 ,lazy:core-true #f)
           (false 0 0 0 ,lazy:core-false #f)
           (lang 1 1 #f ,lazy:core-lang #f)
           (number 0 1 0 ,lazy:core-number #f)
           (sum 1 1 0 ,lazy:core-sum #f)
           (floor 1 1 0 ,lazy:core-floor #f)
           (ceiling 1 1 0 ,lazy:core-ceiling #f)
           (round 1 1 0 ,lazy:core-round #f))))
    (cond
      ((not (eq? (caadr op) 'function-name))
       (draft:signal-semantic-error "not an FunctionName - " (cadr op)))
      ((assq (string->symbol (cadadr op)) core-alist)       
       => (lambda (description)  ; Core function found
            (cond
              ((< (length (cddr op)) (cadr description))
               (draft:signal-semantic-error
                "too few arguments for the Core Function call - "
                (cadadr op)))
              ((and (>= (caddr description) 0)
                    (> (length (cddr op)) (caddr description)))
               (draft:signal-semantic-error
                "too many arguments for the Core Function call - "
                (cadadr op)))
              (else  ; correct number of arguments
               (and-let*
                ((args-impl (lazy:ast-function-arguments (cddr op))))
                (list
                 ; Producing a function implementation
                 (apply (list-ref description 4) num-anc args-impl)
                 (list-ref description 3)
                 (list-ref description 5)  ; requires-last?
                 ))))))
           (else  ; function definition not found
            (draft:signal-semantic-error
             "function call to an unknown function - " (cadadr op))))))

; {22a} ( (argument  <Expr> )* )
; Returns: (listof expr-impl) or #f
(define (lazy:ast-function-arguments op-lst)
  (let ((arg-res-lst
         (map
          (lambda (op)
            (if
             (not (eq? (car op) 'argument))
             (draft:signal-semantic-error "not an Argument - " op)
             (lazy:ast-expr (cadr op) 0)))
          op-lst)))
    (if
     (member #f arg-res-lst)  ; semantic error detected
     #f
     (map car arg-res-lst))))


;=========================================================================
; Highest level API functions
; The API is identical to the API of a context-based SXPath (here we even use
; API helpers from "xpath-context.scm"). For convenience, below we repeat
; comments for the API (borrowed from "xpath-context.scm")
;
; xpath-string - an XPath location path (a string)
; ns+na - can contain 'ns-binding' and/or 'num-ancestors' and/or none of them
; ns-binding - declared namespace prefixes (an optional argument)
;  ns-binding ::= (listof (prefix . uri))
;  prefix - a symbol
;  uri - a string
; num-ancestors - number of ancestors required for resulting nodeset. Can
;  generally be omitted and is than defaulted to 0, which denotes a _usual_
;  nodeset. If a negative number, this signals that all ancestors should be
;  remembered in the context
;
; Returns: (lambda (nodeset position+size var-binding) ...)
; position+size - the same to what was called 'context' in TXPath-1
; var-binding - XPath variable bindings (an optional argument)
;  var-binding = (listof (var-name . value))
;  var-name - (a symbol) a name of a variable
;  value - its value. The value can have the following type: boolean, number,
;  string, nodeset. NOTE: a node must be represented as a singleton nodeset

; Helper for constructing several highest-level API functions
(define (lazy:api-helper grammar-parser ast-parser)
  (lambda (xpath-string . ns+na)
    (call-with-values
     (lambda () (draft:arglist->ns+na ns+na))
     (lambda (ns-binding num-anc)
       (and-let*
        ((ast (grammar-parser xpath-string ns-binding))
         (impl-lst (ast-parser ast num-anc)))
        (let ((query-impl (car impl-lst)))
          (lambda (node . var-binding)
            (let ((query-res
                   (query-impl
                    (as-nodeset node) (cons 1 1)
                    (if (null? var-binding) var-binding (car var-binding)))))
              (if
               (and num-anc (zero? num-anc) (nodeset? query-res))
               (lazy:map sxml:context->node query-res)
               query-res)))))))))

(define lazy:txpath (lazy:api-helper txp:xpath->ast lazy:ast-location-path))
(define lazy:xpath-expr (lazy:api-helper txp:expr->ast lazy:ast-expr))

; Support for native sxpath syntax
(define lazy:sxpath (lazy:api-helper txp:sxpath->ast lazy:ast-expr))